The present invention relates to a new and improved construction of a circuit for an infrared intrusion detector--also sometimes referred to in the art as an infrared radiation-burglary detector--of the type comprising a plurality of receiving directions or receiving regions which are separated from one another and a radiation receiver, at the output of which there appears an electrical signal in accordance with the infrared radiation which is absorbed from all of the receiving directions or receiving regions.
With detectors of this general type, exemplary constructions of which have been disclosed for instance in German petty Pat. Nos. G 76.15724 and G. 76.16785, the disclosures of which are incorporated herein by reference, it is possible to detect the presence of an object, for instance the entry of an unauthorized person or burglar, in a supervised room or area by detecting the infrared radiation which is transmitted by such object or otherwise. What may be detected in this regard is, for instance, the inherent or self-radiation of the person, which lies in a range between 5 and 20.mu., preferably between 7 and 14.mu.. Instead of detecting the self-radiation it is also possible to however provide a source of infrared radiation and to evaluate the radiation which is reflected by the object or the human being. In each instance, the employed optical components must be accommodated or matched to the evaluated wavelength range, i.e. must possess adequate permeability for the employed infrared radiation and the reflector must have a sufficient reflection capability for infrared radiation.
In practice it is necessary to be able to detect even the slight movements of a person in a supervised room or area. Since the total radiation only slightly varies, it has been found to be advantageous to monitor the supervised room or area by covering the same with a number of separate viewing fields or receiving regions with intermediately dispositioned darkened zones or fields. A human being who is in motion, for instance typically a burglar, is thus forced, during the course of his or her movement, to pass a number of times through the boundary or interface between a receiving region and a dark zone or field. Consequently, the output signal of the radiation receiver varies, and such receiver or receiver component, with the heretofore known detectors, can contain either an infrared sensor which is common to all of the receiving regions or a number of separate sensors in an addition circuit.
The evaluation circuits of state-of-the-art detectors containing a number of mutually separated receiving regions employ an alternating-current amplifier constructed as a bandpass for the purpose of amplifying the signals transmitted by the radiation receiver. The frequency range of such amplifier is tuned to the typical or normally encountered speeds of movement of a burglar or the object to be detected, for instance in a range between 0.2 and 3 Hz. Yet, with such type circuit an intruder or burglar only then can be however detected if the alternating-current signal prevails for a predetermined period of time, i.e. if there have been traversed in succession a sufficiently large number of receiving region-boundaries. An instrusion detector containing such an evaluation circuit can be, however, fooled by a burglar or other individual if he or she carries out particularly careful and slow movements. Hence, the reliability of such equipment in practice is too limited.
However, in order to overcome this drawback there have become known to the art also evaluation circuits which already trigger an alarm signal when the output voltage of the amplifier exceeds or falls below a predetermined threshold value only a single time, i.e. when there has been traversed only one boundary of a receiving region. Yet, what is disadvantageous with this solution is that the bandpass-amplifier must possess a very low frequency response or characteristic, in order to be also able to detect slow movements. This requirement is associated, on the one hand, with high technical expenditure in the equipment, and, on the other hand, brings with it the danger that in the presence of changes in the natural ambient or surrounding conditions it is equally possible to trigger an alarm signal. Additionally, individual spurious signals of sufficiently large amplitude, which in an optical or electrical manner can impinge the detector from externally thereof or can be generated by the detector itself, can produce false alarms.